Today's cellular networks are single-vendor systems with proprietary switch/cell-site interfaces that constrain network operators to grow by purchasing equipment from the same vendor that provided their initial equipment. With the recent growth of cellular services, network operators can augment the load capacity of their cellular system by either having the original vendor upgrade the existing system or by having another vendor install a new more powerful system which replaces the existing system.
Since the interface of a base station to a Mobile Telephone Switching Office (MTSO) is proprietary to a vendor, the introduction of a new system by another vendor often requires the installation of new base stations and switching offices to replace existing equipment. Cellular network operators in major service areas are reluctant to completely replace their existing vendor's equipment with a new vendor's equipment. The network operators perceive that a complete replacement could be too costly and too risky to undertake. Further, their system would remain a single vendor network. In order to make the transition as cost effective as possible, the new system must therefore operate in tandem with the old system. This is accomplished by having the new network overlay the old one.
However, since the two networks are incompatible, some problems can arise with call handling between subscribers. Each switching office will often have to handle calls according to the source and destination of each call and of course, the type of subscriber.
Thus, if a new vendor's equipment is to co-exist with the original vendor's equipment, subscribers must perceive no significant reduction in service quality or availability and cellular network operators must not be required to replace undepreciated capital equipment where it can be avoided.
The problem of increasing the load capacity of an existing network is addressed in U.S. Pat. No. 4,799,253 which issued to Stern et al. In this invention, the existing cellular system is interconnected with a new cellular system to provide additional switching capacity to mobiles located in the same geographical area. The existing system has a control terminal coupled to the base stations, to the new control terminal via a one-way inter-office trunk and to a central office via an out trunk. The new control system is coupled to new base stations in the same geographical area as the existing base stations. The new control system is coupled to the central office via in and out trunks. Paging/Access channels are assigned to all new base stations, whereas the existing base stations are assigned Access only channels. The load of mobile originated calls is shared between the two systems by sending a percentage of mobile originated calls to the existing system and remaining calls to the new system.
The problem with this approach, is that the switches are only used to offload mobile originated calls to the new switch. That is, all calls directed to the mobiles are handled by the new switch and all pages transmitted and received to and from the mobiles are processed by the new switch. The result is that the new switch can be constrained with a large processing load, whereas the existing switch may become underutilized.
In addition, a certain limitation can exist with this approach. In particular, since both switches may from time to time serve a given mobile, signalling for vertical features must be identical on both switches to avoid subscriber confusion. For example, certain features such as the flash sequences for conference calling can be problematic. Cellular feature access codes and keystroke procedures should be identical to the landline (Bellcore) standards. For example, "*xy" should mean "call forward all calls to the following DN" in any network.
Accordingly, it is an object of the present invention to provide to a network operator a method of increasing the capacity of an existing network by operating a new cellular system in tandem with an existing system without disrupting service to existing subscribers.
Another object of the present invention is to provide a network operator with a method of balancing load between an existing network and a new network.